Immersion lithography may offer better resolution enhancement and higher numerical apertures at a given exposure wavelength over conventional projection lithography. For example, immersion lithography could extend lithography at the 193 nm wavelength down to the 45 nm node and below thereby providing an alternative to 157 nm exposure wavelengths, extreme ultraviolet (EUV), and other potential technologies.
The minimum feature width (W) that may be printed with an optical lithography system is determined by the Rayleigh equation: W=(k1λ)/(NA) where k1 is the resolution factor, λ is the wavelength of the exposing radiation and NA is the numerical aperture. Numerical aperture (NA) is determined using the equation: NA=n sin α where n is the index of refraction of the medium surrounding the lens and α is the acceptance angle of the lens. The physical limit to NA for exposure systems using air as a medium between the lens and the wafer is 1. Air is the worst medium because its index of refraction may cause a relatively high amount of bending when light leaves the glass. Since the index of refraction for water and glass is approximately 1.44 and 1.52 respectively, far less bending occurs thereby providing a sharper and deeper focus.
In immersion lithography, the space between the lens and the substrate is filled with a liquid, referred to herein as an immersion fluid, that has a refractive index greater than 1. The immersion fluid should preferably exhibit a low optical absorption at the operating wavelength such as, for example 193 nm and 157 nm, be compatible with the photoresist and the lens material, be uniform and be non-contaminating. A preferred immersion fluid for 193 nm immersion lithography is ultra pure water. Ultra pure water has an index of refraction of approximately 1.44, exhibits absorption of less than 5% at working distances of up to 6 mm, is compatible with photoresist and lens, and is non-contaminating in its ultra pure form. Applying the Rayleigh equation using n=1.44 and assuming sin α can reach 0.93, the feature width for 193 nm could reach a theoretical minimum resolution of 36 nm. Still other immersion fluids that have been considered for 15 nm immersion lithography are KRYTOX™ and perfluoropolyether (PFPE).
To date, immersion lithography has not been widely implemented in commercial semiconductor processing partly because improvements in resolution by conventional methods have been possible, but also partly because of practical limitations in implementing immersion lithography. The wafer stage of a typical 193 nm exposure tool steps from location to location across the wafer scanning the reticle image for each field. In order to achieve high throughput, the stage should accelerate rapidly, move accurately to the next field location, settle, scan the image, and then step to the next location within a short time interval. The immersion fluid is typically introduced between the lens and the resist surface of the substrate using a jet stream of the immersion fluid. The space between the lens and the resist surface, referred to herein as the working distance, is less than 6 mm or typically 1 mm. Due to a variety of factors such as short process cycle time, minimal working distance, and dynamics of the immersion stream, maintaining a consistent bubble free liquid between the lens and the resist-coated wafer is very difficult. Further, there is a lack of immersion fluids that have appropriate optical transmission characteristics and chemical compatibility with lithographic systems.
The desire to develop immersion systems is growing more acute because the ability to achieve resolution improvements via conventional means, such as wavelength reduction, appears to be increasingly difficult, particularly at wavelengths below 365 nm. In addition, with numerical apertures or NAs produced by lithographic methods using air as the immersion medium approaching the theoretical limit, progress using conventional methods is bounded. Accordingly, there is a need for an immersion fluid that is compatible with immersion lithographic systems, particularly those systems having an operative wavelength below 365 nm.